Cracking hydrocarbon feedstock containing resid utilizing partial condensation of vapor phase from vapor/liquid separation to mitigate fouling in a flash/separation vessel

ABSTRACT

A process is provided for cracking hydrocarbon feedstock containing resid comprising: heating the feedstock, mixing the heated feedstock with a fluid and/or a primary dilution steam stream to form a mixture, optionally further heating the mixture, flashing the mixture within a flash/separation vessel to form a vapor phase and a liquid phase, partially condensing the vapor phase by contacting with a condenser within the vessel, to condense at least some coke precursors within the vapor while providing condensates which add to the liquid phase, removing the vapor phase of reduced coke precursors content as overhead and the liquid phase as bottoms, heating the vapor phase, cracking the vapor phase in a radiant section of a pyrolysis furnace to produce an effluent comprising olefins, and quenching the effluent and recovering cracked product therefrom. An apparatus for carrying out the process is also provided.

FIELD OF THE INVENTION

The present invention relates to the cracking of hydrocarbons thatcontain relatively non-volatile hydrocarbons, e.g., resids, and othercontaminants. More particularly, the present invention relates to thereduction of fouling during operation caused by coke precursors presentin vapor phase overheads.

BACKGROUND

Steam cracking, also referred to as pyrolysis, has long been used tocrack various hydrocarbon feedstocks into olefins, preferably lightolefins such as ethylene, propylene, and butenes. Conventional steamcracking utilizes a pyrolysis furnace which has two main sections: aconvection section and a radiant section. The hydrocarbon feedstocktypically enters the convection section of the furnace as a liquid(except for light feedstocks which enter as a vapor) wherein it istypically heated and vaporized by indirect contact with hot flue gasfrom the radiant section and by direct contact with steam. The vaporizedfeedstock and steam mixture is then introduced into the radiant sectionwhere the cracking takes place. The resulting products, includingolefins, leave the pyrolysis furnace for further downstream processing,including quenching.

Pyrolysis involves heating the feedstock sufficiently to cause thermaldecomposition of the larger molecules. The pyrolysis process, however,produces molecules which tend to combine to form high molecular weightmaterials known as tar. Tar is a high-boiling point, viscous, reactivematerial that can foul equipment under certain conditions. In general,feedstocks containing higher boiling materials tend to produce greaterquantities of tar.

The formation of tar after the pyrolysis effluent leaves the steamcracking furnace can be minimized by rapidly reducing the temperature ofthe effluent exiting the pyrolysis unit to a level at which thetar-forming reactions are greatly slowed. This cooling, which may beachieved in one or more steps and using one or more methods, is referredto as quenching.

Conventional steam cracking systems have been effective for crackinghigh-quality feedstock which contains a large fraction of light volatilehydrocarbons, such as gas oil and naphtha. However, steam crackingeconomics sometimes favor cracking lower cost feedstocks containingresids such as, by way of non-limiting examples, atmospheric residue(e.g. atmospheric pipestill bottoms) and crude oil. Crude oil andatmospheric residue often contain high molecular weight, non-volatilecomponents with boiling points in excess of 590° C. (1100° F.). Thenon-volatile, components of these feedstocks lay down as coke in theconvection section of conventional pyrolysis furnaces. Only very lowlevels of non-volatile components can be tolerated in the convectionsection downstream of the point where the lighter components have fullyvaporized.

Cracking heavier feeds, such as kerosenes and gas oils, produces largeamounts of tar, which leads to rapid coking in the radiant section ofthe furnace as well as fouling in the transfer line exchangers preferredin lighter liquid cracking service.

Additionally, during transport, some naphthas are contaminated withheavy crude oil containing non-volatile components. Conventionalpyrolysis furnaces do not have the flexibility to process residues,crudes, or many residue or crude contaminated gas oils or naphthas whichare contaminated with non-volatile components.

To address coking problems, U.S. Pat. No. 3,617,493, which isincorporated herein by reference, discloses the use of an externalvaporization drum for the crude oil feed and discloses the use of afirst flash to remove naphtha as vapor and a second flash to removevapors with a boiling point between 450 and 1100° F. (230 and 590° C.).The vapors are cracked in the pyrolysis furnace into olefins, and theseparated liquids from the two flash tanks are removed, stripped withsteam, and used as fuel.

U.S. Pat. No. 3,718,709, which is incorporated herein by reference,discloses a process to minimize coke deposition. It describes preheatingof heavy feedstock inside or outside a pyrolysis furnace to vaporizeabout 50% of the heavy feedstock with superheated steam and the removalof the residual, separated liquid. The vaporized hydrocarbons, whichcontain mostly light volatile hydrocarbons, are subjected to cracking.

U.S. Pat. No. 5,190,634, which is incorporated herein by reference,discloses a process for inhibiting coke formation in a furnace bypreheating the feedstock in the presence of a small, critical amount ofhydrogen in the convection section. The presence of hydrogen in theconvection section inhibits the polymerization reaction of thehydrocarbons thereby inhibiting coke formation.

U.S. Pat. No. 5,580,443, which is incorporated herein by reference,discloses a process wherein the feedstock is first preheated and thenwithdrawn from a preheater in the convection section of the pyrolysisfurnace. This preheated feedstock is then mixed with a predeterminedamount of steam (the dilution steam) and is then introduced into agas-liquid separator to separate and remove a required proportion of thenon-volatiles as liquid from the separator. The separated vapor from thegas-liquid separator is returned to the pyrolysis furnace for heatingand cracking.

Co-pending U.S. application Ser. No. 10/188,461 filed Jul. 3, 2002,Patent Application Publication U.S. 2004/0004022 A1, published Jan. 8,2004, which is incorporated herein by reference, describes anadvantageously controlled process to optimize the cracking of volatilehydrocarbons contained in the heavy hydrocarbon feedstocks and to reduceand avoid coking problems. It provides a method to maintain a relativelyconstant ratio of vapor to liquid leaving the flash by maintaining arelatively constant temperature of the stream entering the flash. Morespecifically, the constant temperature of the flash stream is maintainedby automatically adjusting the amount of a fluid stream mixed with theheavy hydrocarbon feedstock prior to the flash. The fluid can be water.

Co-pending U.S. Patent Application Ser. No. 60/555,282, filed Mar. 22,2004, (Attorney Docket 2004B001-US), which is incorporated herein byreference, describes a process for cracking heavy hydrocarbon feedstockwhich mixes heavy hydrocarbon feedstock with a fluid, e.g., hydrocarbonor water, to form a mixture stream which is flashed to form a vaporphase and a liquid phase, the vapor phase being subsequently cracked toprovide olefins, with product effluent cooled in a transfer lineexchanger, wherein the amount of fluid mixed with the feedstock isvaried in accordance with a selected operating parameter of the process,e.g., temperature of the mixture stream before the mixture stream isflashed.

Co-pending U.S. patent application Ser. No. ______, filed herewith,(Attorney Docket 2004B042-US; PM2004-051; RMH11695), which isincorporated herein by reference, describes a process for cracking heavyhydrocarbon feedstock which mixes heavy hydrocarbon feedstock with afluid, e.g., hydrocarbon or water, to form a mixture stream which isflashed to form a vapor phase and a liquid phase, the vapor phase beingsubsequently cracked to provide olefins. Fouling downstream of theflash/separation vessel is reduced by superheating the vapor in theupper portion of the vessel. A condenser may also be utilized within thevessel to improve liquid/vapor separation.

In using a flash to separate heavy liquid hydrocarbon fractionscontaining resid from the lighter fractions which can be processed inthe pyrolysis furnace, it is important to effect the separation so thatmost of the non-volatile components will be in the liquid phase.Otherwise, heavy, coke-forming non-volatile components in the vapor arecarried into the furnace causing coking problems.

Increasing the cut in the flash drum, or the fraction of the hydrocarbonthat vaporizes, is also extremely desirable because resid-containingliquid hydrocarbon fractions generally have a low value, often less thanheavy fuel oil. Vaporizing some of the heavier fractions produces morevaluable steam cracker feed. Although this can be accomplished byincreasing the flash drum temperature to increase the cut, the resultingheavier fractions thus vaporized tend to condense once the overheadvapor phase leaves the flash drum, resulting in fouling of the lines andvessels downstream of the flash drum overhead outlet.

Accordingly, it would be desirable to provide a process for treatingvapor phase materials within a flash drum to remove components which aresusceptible to condensing downstream of the drum overheads outlet.

SUMMARY

In one aspect, the present invention relates to a process for cracking ahydrocarbon feedstock containing resid. The process comprises: (a)heating the hydrocarbon feedstock; (b) mixing the heated hydrocarbonfeedstock with steam to form a mixture stream; (c) introducing themixture stream in a flash/separation vessel through an inlet to form i)a vapor phase at its dew point which contains a lesser portion of cokeprecursors and ii) a liquid phase which contains a greater portion ofcoke precursors; (d) partially condensing the vapor phase within theflash/separation vessel by contacting the vapor phase with a condenser,which condenses at least some of the lesser portion of coke precursors,which adds to the liquid phase, the condensing providing a vapor phaseabove the condenser of reduced coke precursors content; (e) removing thevapor phase of reduced coke precursors content as overhead through anoverhead outlet, and the liquid phase as bottoms, from theflash/separation vessel; (f) heating the vapor phase; (g) cracking theheated vapor phase in a radiant section of a pyrolysis furnace toproduce an effluent comprising olefins, the pyrolysis furnace comprisinga radiant section and a convection section; and (h) quenching theeffluent and recovering cracked product therefrom. In one embodiment,the mixture stream is heated prior to introduction to theflash/separation vessel.

In another aspect, the present invention relates to a flash/separationvessel for treating hydrocarbon feedstock containing resid to provide aliquid phase and a vapor phase, which comprises: (A) an inlet forintroducing to the vessel under flashing conditions a mixture streamcomprising the hydrocarbon feedstock and steam where the mixture streamundergoes an initial flashing to form i) a vapor phase at its dew pointwhich contains a lesser portion of coke precursors; and ii) a liquidphase which contains a greater portion of coke precursors; (B) a partialcondenser for contacting the vapor phase within the flash/separationvessel and at least partially condensing at least some of the lesserportion of coke precursors, which adds to the liquid phase, thecondensing providing a vapor phase of reduced coke precursor content;(C) a flash/separation vessel overhead outlet for removing the vaporphase of reduced coke precursors content as overhead; and (D) aflash/separation vessel liquid outlet for removing the liquid phase asbottoms from the flash/separation vessel.

In still another aspect, the present invention relates to an apparatusfor cracking a hydrocarbon feedstock containing resid, the apparatuscomprising: (a) a heating zone for heating the hydrocarbon feedstock toprovide heated hydrocarbon feedstock; (b) a mixing zone for mixing aprimary dilution steam stream with the heated hydrocarbon feedstock toprovide a heated two-phase stratified open channel flow mixture stream;(c) a vapor/liquid separation zone for treating vapor/liquid mixtures ofhydrocarbons and steam, the separation zone comprising: (i) asubstantially cylindrical vertical drum having an upper cap section, amiddle section comprising a circular wall, and a lower cap section; (ii)an overhead vapor outlet attached to the upper cap section; (iii) atleast one substantially tangentially positioned inlet in the wall of themiddle section for introducing the flow mixture stream along the wallunder flashing conditions where the flow mixture stream undergoes aninitial flashing to form A) a vapor phase at its dew point whichcontains a lesser portion of coke precursors, and B) a liquid phasewhich contains a greater portion of coke precursors; (iv) a partialcondenser for contacting the vapor phase within the drum for at leastpartially condensing at least some of the lesser portion of cokeprecursors, which adds to the liquid phase, the condensing providing avapor phase of reduced coke precursors content; (v) a drum overheadoutlet for removing the vapor phase of reduced precursors content asoverhead; (vi) a drum liquid outlet for removing the liquid phase asbottoms from the drum; and (vii) a substantially concentricallypositioned, substantially cylindrical boot of less diameter than themiddle section, the boot communicating with the lower cap section, andfurther comprising an inlet for quench oil, e.g., recycle quench oil,and a liquid outlet at its lower end; (d) a pyrolysis furnace comprisinga convection section, and a radiant section for cracking the vapor phasefrom the overhead vapor outlet to produce an effluent comprisingolefins; (e) a means for quenching the effluent; and (f) a recoverytrain for recovering cracked product from the quenched effluent. In oneembodiment, the apparatus further comprises a heating zone for heatingthe mixture stream upstream of the flash/separation zone.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a schematic flow diagram of a process in accordancewith the present invention employed with a pyrolysis furnace.

FIG. 2 illustrates a flash/separation apparatus of the present inventioncomprising dual finned serpentine cooling coils with interposed sheds.

FIG. 3 illustrates a flash/separation apparatus of the present inventionshowing a single parallel finned cooling coil with concentric pipecoolers.

FIG. 4 illustrates a cross-section of concentric pipe coolers used inthe present invention.

DETAILED DESCRIPTION

The present invention relates to a process for cracking hydrocarbonfeedstock containing resid comprising heating the feedstock, mixing theheated feedstock with a fluid and/or a primary dilution steam stream toform a mixture stream, and flashing the mixture stream within aflash/separation vessel to form a vapor phase and a liquid phase. Thevapor phase is partially condensed by contacting with a condenser and,optionally, surfaces (sheds) underneath the condenser to improve contactbetween the condensate and the rising vapor, within the vessel, tocondense at least some coke precursors within the vapor while providingcondensates which add to the liquid phase. The vapor phase of reducedcoke precursors content is removed as overhead and the liquid phase isremoved as bottoms. The vapor phase is heated and then cracked in aradiant section of a pyrolysis furnace to produce an effluent comprisingolefins. The resulting effluent is quenched and cracked product isrecovered from the quenched effluent.

The condenser is advantageously located within the flash/separationvessel, typically above the inlet of the flash/separation vessel whichintroduces the hydrocarbonaceous feed to the vessel. The condensercomprises a vapor/liquid contacting surface which is maintained underconditions sufficient to effect condensation of condensable fractionswithin the vapor phase. In one embodiment, the condenser comprises aheat-conducting tube containing a cooling medium. The tube can be madeof any heat conducting material, e.g., metal, which complies with localboiler and piping codes. A cooling medium is present within the tube,e.g., a fluid such as a liquid or gas. In one embodiment, the coolingmedium comprises liquid, typically, water, e.g., boiler feed water. Thetube typically comprises a tube inlet and a tube outlet for introducingand removing the cooling medium. At least one of the tube inlet and thetube outlet can pass through a wall of the flash/separation vessel, or,alternatively, at least one of the tube inlet and the tube outlet passthrough the overhead outlet of the flash/separation vessel.

In operation, the condenser tube typically has an outside tube metaltemperature (TMT) ranging from about 200 to about 370° C. (400 to 700°F.), say, from about 260 to about 315° C. (500 to 600° F.). At thistemperature, a large amount of hydrocarbon condensation occurs on theoutside of the cooling tubes but not in the drum cross-sectional areabetween the tubes, producing a partial condenser effect. The tube may beof any size sufficient to impart the requisite heat to the vapor phase.Typically, the tube has a diameter of about 10 cm (4 in). For a vesselof about 4 m (13 feet) diameter, the condenser heat duty typicallyranges from about 0.06 to about 0.60 MW (0.2 to 2 MBtu/hr), say, fromabout 0.1 to about 0.3 MW (0.4 to 1 MBtu/hr). In one embodiment, boilerfeed water is passed through the condenser at a rate of about 450 toabout 13000 kg/hr (1 to 30 klb/hr) at a temperature ranging from about100 to about 260° C. (212 to 500° F.), at a pressure ranging from about350 to about 17,000 kpag (50 to 2500 psig).

It is desirable that the condenser fit within the upper portion of theflash/separation vessel; thus the condenser is typically substantiallyplanar and configured so it can be horizontally mounted within thevessel. In one embodiment, the tube present in the condenser iscontinuous and comprised of alternating straight sections and 180° bendsections beginning with a straight inlet section and terminating in astraight outlet section. Cooling medium which is cooler than the vaporphase temperature is introduced via the inlet section and, after heatexchange with the vapor, heated cooling medium is withdrawn through theoutlet section.

In another embodiment, the condenser comprises a substantially straightinlet communicating with an inlet manifold substantially perpendicularto the straight inlet, at least two substantially parallel cooling tubessubstantially perpendicular to and communicating with the inlet manifoldand substantially perpendicular to and communicating with an outletmanifold, and a substantially straight outlet perpendicular to andcommunicating with the outlet manifold.

In one embodiment, the surface area of the tube is enhanced by providingextended surfaces along the tube, e.g., by attaching fins to the tubealong its length. Typically, the tube comprises at least about 2 fin/cmof tube length (5 fins/inch of tube length) and the fins range fromabout ⅝ to about 2½ cm (¼ to 1 in) in height, and about 0.05 to about0.4 cm (0.02 to 0.15 in) in thickness, say, about 2 cm (¾ in) in height,and about ⅛ cm (0.05 in) in thickness.

In still another embodiment, the tube employed in the condensercomprises a substantially concentrically placed inner tube within anouter tube, wherein cooling liquid, e.g., water is passed through theinner tube while steam is passed through the outer tube. Typically, theinner tube has a diameter ranging from about 2½ to about 10 cm (1 to 4in) and the outer tube has a diameter ranging from about 5 to about 15cm (2 to 6 in), say, the inner tube has a diameter of about cm (2 in)and the outer tube has a diameter of about 10 cm (4 in).

In yet another embodiment, a set of passive liquid/vapor contactingsurfaces is positioned beneath the condenser, within theflash/separation vessel. Typically, a set of liquid/vapor contactingsurface(s) is provided by a first row of sheds arranged substantiallyperpendicularly to the tube. The sheds have an inverted V cross-sectionwhich serves to drain liquid formed from the surface downward off thesheds for contacting with the vapor phase or for collection as bottoms.The set of liquid/vapor contacting surfaces can further comprise atleast one additional row of sheds positioned substantially parallel toand beneath the first row of sheds. Other suitable liquid/vaporcontacting surfaces include Glitsch Grid and other distillation towerwide open packing.

In still another embodiment, a second condenser is located beneath theliquid/vapor contacting surfaces to enhance condensation of the vaporphase.

The mixture stream is typically introduced to the flash/separationvessel through an inlet in the side of the flash/separation vessel. Theinlet can be substantially perpendicular to the vessel wall, or moreadvantageously, angled so as to be at least partially tangential to thevessel wall in order to effect swirling of the mixture stream feedwithin the vessel.

The process of the present invention is typically operated so that thecondensing step provides a vapor phase reduced in coke precursor contentby at least about 50%, say at least about 80%, relative to a comparablevapor phase produced in the absence of the condensing.

Quenching the effluent leaving the pyrolysis furnace may be carried outusing a transfer line exchanger, wherein the amount of the fluid mixedwith the hydrocarbon feedstock is varied in accordance with at least oneselected operating parameter of the process. The fluid can be ahydrocarbon or water, preferably water.

In applying this invention, the hydrocarbon feedstock containing residand coke precursors may be heated by indirect contact with flue gas in afirst convection section tube bank of the pyrolysis furnace beforemixing with the fluid. Preferably, the temperature of the hydrocarbonfeedstock is from about 150° C. to about 260° C. (300° F. to 500° F.)before mixing with the fluid.

The mixture stream may then be heated by indirect contact with flue gasin a first convection section of the pyrolysis furnace before beingflashed. Preferably, the first convection section is arranged to add thefluid, and optionally, primary dilution steam, between passes of thatsection such that the hydrocarbon feedstock can be heated before mixingwith the fluid and the mixture stream can be further heated before beingflashed.

The temperature of the flue gas entering the first convection sectiontube bank is generally less than about 815° C. (1500° F.), for exampleless than about 700° C. (1300° F.), such as less than about 620° C.(1150° F.), and preferably less than about 540° C. (1000° F.).

Dilution steam may be added at any point in the process, for example, itmay be added to the hydrocarbon feedstock containing resid before orafter heating, to the mixture stream, and/or to the vapor phase. Anydilution steam stream may comprise sour or process steam. Any dilutionsteam stream may be heated or superheated in a convection section tubebank located anywhere within the convection section of the furnace,preferably in the first or second tube bank.

The mixture stream may be at about 315 to about 540° C. (600° F. to1000° F.) before the flash in step (c), and the flash pressure may beabout 275 to about 1375 kPa (40 to 200 psia). Following the flash, 50 to98% of the mixture stream may be in the vapor phase. An additionalseparator such as a centrifugal separator may be used to remove traceamounts of liquid from the vapor phase. The vapor phase may be heatedabove the flash temperature before entering the radiant section of thefurnace, for example, from about 425 to about 705° C. (800 to 1300° F.).This heating may occur in a convection section tube bank, preferably thetube bank nearest the radiant section of the furnace.

Unless otherwise stated, all percentages, parts, ratios, etc. are byweight. Unless otherwise stated, a reference to a compound or componentincludes the compound or component by itself, as well as in combinationwith other compounds or components, such as mixtures of compounds.

Further, when an amount, concentration, or other value or parameter isgiven as a list of upper preferable values and lower preferable values,this is to be understood as specifically disclosing all ranges formedfrom any pair of an upper preferred value and a lower preferred value,regardless whether ranges are separately disclosed.

As used herein, non-volatile components, or resids, are the fraction ofthe hydrocarbon feed with a nominal boiling point above about 590° C.(1100° F.) as measured by ASTM D-6352-98 or D-2887. This invention worksvery well with non-volatiles having a nominal boiling point above about760° C. (1400° F.). The boiling point distribution of the hydrocarbonfeed is measured by Gas Chromatograph Distillation (GCD) by ASTMD-6352-98 or D-2887 extended by extrapolation for materials boilingabove 700° C. (1292° F.). Non-volatiles include coke precursors, whichare large, condensable molecules that condense in the vapor, and thenform coke under the operating conditions encountered in the presentprocess of the invention.

The hydrocarbon feedstock can comprise a large portion, such as about 2to about 50%, of non-volatile components. Such feedstock could comprise,by way of non-limiting examples, one or more of steam cracked gas oiland residues, gas oils, heating oil, jet fuel, diesel, kerosene,gasoline, coker naphtha, steam cracked naphtha, catalytically crackednaphtha, hydrocrackate, reformate, raffinate reformate, Fischer-Tropschliquids, Fischer-Tropsch gases, natural gasoline, distillate, virginnaphtha, atmospheric pipestill bottoms, vacuum pipestill streamsincluding bottoms, wide boiling range naphtha to gas oil condensates,heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils,heavy gas oil, naphtha contaminated with crude, atmospheric residue,heavy residue, hydrocarbon gases/residue admixtures, hydrogen/residueadmixtures, C4's/residue admixture, naphtha/residue admixture, gasoil/residue admixture, and crude oil.

The hydrocarbon feedstock can have a nominal end boiling point of atleast about 315° C. (600° F.), generally greater than about 510° C.(950° F.), typically greater than about 590° C. (1100° F.), for example,greater than about 760° C. (1400° F.). The economically preferredfeedstocks are generally low sulfur waxy residues, atmospheric residues,naphthas contaminated with crude, various residue admixtures, and crudeoils.

The heating of the hydrocarbon feedstock containing resid can take anyform known by those of ordinary skill in the art. However, as seen inFIG. 1, it is preferred that the heating comprises indirect contact ofthe hydrocarbon feedstock 40 in the upper (farthest from the radiantsection) convection section tube bank 2 of the furnace 1 with hot fluegases from the radiant section of the furnace. This can be accomplished,by way of non-limiting example, by passing the hydrocarbon feedstockthrough a bank of heat exchange tubes 2 located within the convectionsection 3 of the furnace 1. The heated hydrocarbon feedstock typicallyhas a temperature between about 150 and about 260° C. (300 to 500° F.),such as between about 160 to about 230° C. (325 to 450° F.), forexample, between about 170 to about 220° C. (340 to 425° F.).

The heated hydrocarbon feedstock is mixed with primary dilution steamand optionally, a fluid that can be a hydrocarbon (preferably liquid butoptionally vapor), water, steam, or a mixture thereof. The preferredfluid is water. A source of the fluid can be low-pressure boiler feedwater. The temperature of the fluid can be below, equal to, or above thetemperature of the heated feedstock.

The mixing of the heated hydrocarbon feedstock and the fluid can occurinside or outside the pyrolysis furnace 1, but preferably it occursoutside the furnace. The mixing can be accomplished using any mixingdevice known within the art. For example, it is possible to use a firstsparger 4 of a double sparger assembly 9 for the mixing. The firstsparger 4 can avoid or reduce hammering, caused by sudden vaporizationof the fluid, upon introduction of the fluid into the heated hydrocarbonfeedstock.

The present invention typically uses steam streams in various parts ofthe process. The primary dilution steam stream 17 can be mixed with theheated hydrocarbon feedstock as detailed below. In another embodiment, asecondary dilution steam stream 18 can be heated in the convectionsection and mixed with the heated mixture steam before the flash. Thesource of the secondary dilution steam may be primary dilution steamthat has been superheated, optionally, in a convection section of thepyrolysis furnace. Either or both of the primary and secondary dilutionsteam streams may comprise sour or process steam. Superheating the souror process dilution steam minimizes the risk of corrosion, which couldresult from condensation of sour or process steam.

In one embodiment of the present invention, in addition to the fluidmixed with the heated feedstock, the primary dilution steam 17 is alsomixed with the feedstock. The primary dilution steam stream can bepreferably injected into a second sparger 8. It is preferred that theprimary dilution steam stream is injected into the hydrocarbon fluidmixture before the resulting stream mixture optionally enters theconvection section at 11 for additional heating by flue gas, generallywithin the same tube bank as would have been used for heating thehydrocarbon feedstock.

The primary dilution steam can have a temperature greater, lower orabout the same as hydrocarbon feedstock fluid mixture but preferably thetemperature is greater than that of the mixture and serves to partiallyvaporize the feedstock/fluid mixture. The primary dilution steam may besuperheated before being injected into the second sparger 8.

The mixture stream comprising the heated hydrocarbon feedstock, thefluid, and the primary dilution steam stream leaving the second sparger8 is optionally heated again in the convection section of the pyrolysisfurnace 3 before the flash. The heating can be accomplished, by way ofnon-limiting example, bypassing the mixture stream through a bank ofheat exchange tubes 6 located within the convection section, usually aspart of the first convection section tube bank, of the furnace and thusheated by the hot flue gas from the radiant section of the furnace. Thethus-heated mixture stream leaves the convection section as a mixturestream 12 to optionally be further mixed with an additional steamstream.

Optionally, the secondary dilution steam stream 18 can be further splitinto a flash steam stream 19 which is mixed with the hydrocarbon mixture12 before the flash and a bypass steam stream 21 which bypasses theflash of the hydrocarbon mixture and, instead is mixed with the vaporphase from the flash before the vapor phase is cracked in the radiantsection of the furnace. The present invention can operate with allsecondary dilution steam 18 used as flash steam 19 with no bypass steam21. Alternatively, the present invention can be operated with secondarydilution steam 18 directed to bypass steam 21 with no flash steam 19. Ina preferred embodiment in accordance with the present invention, theratio of the flash steam stream 19 to bypass steam stream 21 should bepreferably 1:20 to 20:1, and most preferably 1:2 to 2:1. In thisembodiment, the flash steam 19 is mixed with the hydrocarbon mixturestream 12 to form a flash stream 20, which typically is introducedbefore the flash in flash/separation vessel 5. Preferably, the secondarydilution steam stream is superheated in a superheater section 16 in thefurnace convection before splitting and mixing with the hydrocarbonmixture. The addition of the flash steam stream 19 to the hydrocarbonmixture stream 12 aids the vaporization of most volatile components ofthe mixture before the flash stream 20 enters the flash/separator vessel5.

The mixture stream 12 or the flash stream 20 is then introduced forflashing, either directly or through a tangential inlet (to impartswirl) to a flash/separation apparatus, e.g., flash/separator vessel 5,for separation into two phases: a vapor phase comprising predominantlyvolatile hydrocarbons and steam and a liquid phase comprisingpredominantly non-volatile hydrocarbons. The vapor phase is preferablyremoved from the flash/separator vessel as an overhead vapor stream 13.The vapor phase, preferably, is fed back to a convection section tubebank 23 of the furnace, preferably located nearest the radiant sectionof the furnace, for optional heating and through crossover pipes 24 tothe radiant section of the pyrolysis furnace for cracking. The liquidphase of the flashed mixture stream is removed from the flash/separatorvessel 5 as a bottoms stream 27.

It is preferred to maintain a predetermined constant ratio of vapor toliquid in the flash/separator vessel 5, but such ratio is difficult tomeasure and control. As an alternative, temperature of the mixturestream 12 before the flash/separator vessel 5 can be used as an indirectparameter to measure, control, and maintain an approximately constantvapor to liquid ratio in the flash/separator vessel 5. Ideally, when themixture stream temperature is higher, more hydrocarbons will bevaporized and become available, as a vapor phase, for cracking. However,when the mixture stream temperature is too high, more heavy hydrocarbonswill be present in the vapor phase and carried over to the convectionfurnace tubes, eventually coking the tubes. If the mixture stream 12temperature is too low, resulting in a low ratio of vapor to liquid inthe flash/separator vessel 5, more volatile hydrocarbons will remain inliquid phase and thus will not be available for cracking.

The mixture stream temperature is limited by highestrecovery/vaporization of volatiles in the feedstock while avoidingexcessive coking in the furnace tubes or coking in piping and vesselsconveying the mixture from the flash/separator vessel to the furnace 1via line 13. The pressure drop across the vessels and piping 13conveying the mixture to the lower convection section 23, and thecrossover piping 24, and the temperature rise across the lowerconvection section 23 may be monitored to detect the onset of cokingproblems. For instance, when the crossover pressure and process inletpressure to the lower convection section 23 begins to increase rapidlydue to coking, the temperature in the flash/separator vessel 5 and themixture stream 12 should be reduced. If coking occurs in the lowerconvection section, the temperature of the flue gas to the superheater16 increases, requiring more desuperheater water 26 via valve 25.

The selection of the mixture stream 12 temperature is also determined bythe composition of the feedstock materials. When the feedstock containshigher amounts of lighter hydrocarbons, the temperature of the mixturestream 12 can be set lower. As a result, the amount of fluid used in thefirst sparger 4 would be increased and/or the amount of primary dilutionsteam used in the second sparger 8 would be decreased since theseamounts directly impact the temperature of the mixture stream 12. Whenthe feedstock contains a higher amount of non-volatile hydrocarbons, thetemperature of the mixture stream 12 should be set higher. As a result,the amount of fluid used in the first sparger 4 would be decreased whilethe amount of primary dilution steam used in the second sparger 8 wouldbe increased. By carefully selecting a mixture stream temperature, thepresent invention can find applications in a wide variety of feedstockmaterials.

Typically, the temperature of the mixture stream 12 can be set andcontrolled at between about 315 and about 540° C. (600 and 1000° F.),such as between about 370 and about 510° C. (700 and 950° F.), forexample, between about 400 and about 480° C. (750 and 900° F.), andoften between about 430 and about 475° C. (810 and 890° F.). Thesevalues will change with the concentration of volatiles in the feedstockas discussed above.

Considerations in determining the temperature include the desire tomaintain a liquid phase to reduce the likelihood of coke formation onexchanger tube walls and in the flash/separator.

The temperature of mixture stream 12 can be controlled by a controlsystem 7 which comprises at least a temperature sensor and any knowncontrol device, such as a computer application. Preferably, thetemperature sensors are thermocouples. The control system 7 communicateswith the fluid valve 14 and the primary dilution steam valve 15 so thatthe amount of the fluid and the primary dilution steam entering the twospargers can be controlled.

In order to maintain a constant temperature for the mixture stream 12mixing with flash steam 19 and entering the flash/separator vessel toachieve a constant ratio of vapor to liquid in the flash/separatorvessel 5, and to avoid substantial temperature and flash vapor to liquidratio variations, the present invention operates as follows: When atemperature for the mixture stream 12 before the flash/separator vessel5 is set, the control system 7 automatically controls the fluid valve 14and primary dilution steam valve 15 on the two spargers. When thecontrol system 7 detects a drop of temperature of the mixture stream, itwill cause the fluid valve 14 to reduce the injection of the fluid intothe first sparger 4. If the temperature of the mixture stream starts torise, the fluid valve will be opened wider to increase the injection ofthe fluid into the first sparger 4. In one possible embodiment, thefluid latent heat of vaporization controls mixture stream temperature.

When the primary dilution steam stream 17 is injected to the secondsparger 8, the temperature control system 7 can also be used to controlthe primary dilution steam valve 15 to adjust the amount of primarydilution steam stream injected to the second sparger 8. This furtherreduces the sharp variation of temperature changes in the flash 5. Whenthe control system 7 detects a drop of temperature of the mixture stream12, it will instruct the primary dilution steam valve 15 to increase theinjection of the primary dilution steam stream into the second sparger 8while valve 14 is closed more. If the temperature starts to rise, theprimary dilution steam valve will automatically close more to reduce theprimary dilution steam stream injected into the second sparger 8 whilevalve 14 is opened wider.

In one embodiment in accordance with the present invention, the controlsystem 7 can be used to control both the amount of the fluid and theamount of the primary dilution steam stream to be injected into bothspargers.

In an example embodiment where the fluid is water, the controller variesthe amount of water and primary dilution steam to maintain a constantmixture stream temperature 12, while maintaining a constant ratio ofH₂O-to-feedstock in the mixture 11. To further avoid sharp variation ofthe flash temperature, the present invention also preferably utilizes anintermediate desuperheater providing desuperheater water 26 via valve 25to the superheating section 16 of the secondary dilution steam in thefurnace. This allows the superheater outlet temperature to be controlledat a constant value, independent of furnace load changes, coking extentchanges, excess oxygen level changes, and other variables. Normally,this desuperheater maintains the temperature of the secondary dilutionsteam between about 425 and about 590° C. (800 and 1100° F.), forexample, between about 455 and about 540° C. (850 and 1000° F.), such asbetween about 455 and about 510° C. (850 and 950° F.), and typicallybetween about 470 and about 495° C. (875 and 925° F.). The desuperheatercomprises the control valve 25 and an optional water atomizer nozzle.After partial preheating, the secondary dilution steam exits theconvection section and a fine mist of water can be added which rapidlyvaporizes and reduces the temperature. The steam is preferably thenfurther heated in the convection section. The amount of water added tothe superheater can control the temperature of the steam mixed withmixture stream 12.

Although the description above is based on adjusting the amounts of thefluid and the primary dilution steam streams injected into thehydrocarbon feedstock in the two spargers 4 and 8, according to thepredetermined temperature of the mixture stream 12 before theflash/separator vessel 5, the same control mechanisms can be applied toother parameters at other locations. For instance, the flash pressureand the temperature and the flow rate of the flash steam 19 can bechanged to effect a change in the vapor to liquid ratio in the flash.Also, excess oxygen in the flue gas can also be a control variable,albeit possibly a slow one.

In addition to maintaining a constant temperature of the mixture stream12 entering the flash/separator vessel, it is generally also desirableto maintain a constant hydrocarbon partial pressure of the flash stream20 in order to maintain a constant ratio of vapor to liquid in theflash/separator vessel. By way of examples, the constant hydrocarbonpartial pressure can be maintained by keeping constant flash/separatorvessel pressure through the use of control valves 36 on the vapor phaseline 13, and by controlling the ratio of steam to hydrocarbon feedstockin stream 20.

Typically, the hydrocarbon partial pressure of the flash stream in thepresent invention is set and controlled at between about 25 and about175 kPa (4 and about 25 psia), such as between about 35 and about 100kPa (5 and 15 psia), for example, between about 40 and about 75 kPa (6and 11 psia).

In one embodiment, the flash is conducted in at least oneflash/separator vessel. Typically the flash is a one-stage process withor without reflux. The flash/separator vessel 5 is normally operated atabout 275 to 1400 kPa (40 to 200 psia) pressure and its temperature isusually the same or slightly lower than the temperature of the flashstream 20 at the flash/separation apparatus feed inlet before enteringthe flash/separator vessel 5. Typically, the pressure at which theflash/separator vessel operates is at about 275 to about 1400 kPa (40 to200 psia) and the temperature is at about 310 to about 540° C. (600 to1000° F.). For example, the pressure of the flash can be about 600 toabout 1100 kPa (85 to 160 psia) and the temperature can be about 370 toabout 490° C. (700 to 920° F.). As a further example, the pressure ofthe flash can be about 700 to about 1000 kPa (100 to 145 psia) with atemperature of about 400 to about 480° C. (750 to 900° F.). In yetanother example, the pressure of the flash/separator vessel can be about700 to about 860 kPa (100 to 125 psia) and the temperature can be about430 to about 475° C. (810 to 890° F.). Depending on the temperature ofthe mixture stream 12, generally about 50 to about 98% of the mixturestream being flashed is in the vapor phase, such as about 60 to about95%, for example, about 65 to about 90%.

The flash/separator vessel 5 is generally operated, in one aspect, tominimize the temperature of the liquid phase at the bottom of the vesselbecause too much heat may cause coking of the non-volatiles in theliquid phase. Use of the secondary dilution steam stream 18 in the flashstream entering the flash/separator vessel lowers the vaporizationtemperature because it reduces the partial pressure of the hydrocarbons(i.e., a larger mole fraction of the vapor is steam) and thus lowers therequired liquid phase temperature. It may also be helpful to recycle aportion of the externally cooled flash/separator vessel bottoms liquid30 back to the flash/separator vessel to help cool the newly separatedliquid phase at the bottom of the flash/separator vessel 5. Stream 27can be conveyed from the bottom of the flash/separator vessel 5 to thecooler 28 via pump 37. The cooled stream 29 can then be split into arecycle stream 30 and export stream 22. The temperature of the recycledstream would typically be about 260 to about 315° C. (500 to 600° F.),for example, about 270 to about 290° C. (520 to 550° F.). The amount ofrecycled stream can be from about 80 to about 250% of the amount of thenewly separated bottom liquid inside the flash/separator vessel, such asfrom about 90 to about 225%, for example, from about 100 to about 200%.

The flash is generally also operated, in another aspect, to minimize theliquid retention/holding time in the flash vessel. In one exampleembodiment, the liquid phase is discharged from the vessel through asmall diameter “boot” or cylinder 35 on the bottom of theflash/separator vessel. Typically, the liquid phase retention time inthe drum is less than about 75 seconds, for example, less than about 60seconds, such as less than about 30 seconds, and often less than about15 seconds. The shorter the liquid phase retention/holding time in theflash/separator vessel, the less coking occurs in the bottom of theflash/separator vessel.

When the mixture of steam and water mixed with hydrocarbon enters theflash/separator vessel 5, a perfect or near perfect vapor/liquidseparation occurs, with the vapor being at its dew point. Since theflash drum has no theoretical stages of separation, even if thevapor/liquid separation is perfect, thermodynamic calculations predictabout 10 ppm of the hydrocarbon vapor has a normal boiling point above760° C. (1400° F.). The vapor spends about 30 seconds in the flash drumat 450° C. (850° F.) causing cracking and coking of some of the heaviermolecules. Because cracking and coking are endothermic reactions, thevapor will cool below its dew point, causing a fraction of the heaviermolecules to condense. Coking of the condensed molecules produces evenheavier molecules and the condensed and coked molecules foul the pipingdownstream from the overheads outlet of the flash drum, e.g., the pipingdownstream of centrifugal separator 38 and crossover piping 24.Accordingly, the present invention treats the vapor phase by contactingit with condenser 104 to effect partial condensation of the vapor phase.

In one embodiment, as depicted in FIG. 2, the feed mixture containinghydrocarbon and steam is introduced through a tangential inlet 120 vialine 20. The condenser 104 comprises a first serpentine, finned coolingcoil 112 inside the top of the flash/separator vessel 5 which coil has acooling medium inlet 108, and a cooling medium outlet 110. The finseffect good drop distribution across the flash/separator vesselcross-section area as compared to bare tubes. Droplets forming on thecoil and fins can flow down the fin surface, improving vapor/liquid heatand mass transfer. Sheds 106 are installed below the first coil. Asecond serpentine finned cooling coil 114 having a cooling medium inlet116 and a cooling medium outlet 118 is installed beneath the sheds.Hydrocarbon liquid drops fall off the sheds into the boot 35 preventingcoke buildup.

In another embodiment, as depicted in FIG. 3, the feed mixturecontaining hydrocarbon and steam is introduced through a tangentialinlet 120 via line 20. The condenser 130 can comprise a substantiallystraight inlet 132 communicating with an inlet manifold 134 and parallelcooling tubes 136 substantially perpendicular to and communicating withinlet manifold 134 and substantially perpendicular to and communicatingwith an outlet manifold 138, with a substantially straight outlet 140perpendicular to and communicating with the outlet manifold.

In one embodiment, the cooling tubes 136 comprise concentric pipes asdepicted in FIG. 4, with an internal pipe 142 through which water 144 ispassed and a concentric external pipe 146 through which steam 148 ispassed. This arrangement permits a reduced water rate. Water flowsthrough the inner pipe while low pressure steam flows through theannulus. Because low pressure steam has a relatively low thermalconductivity, the tube metal temperature of the outside pipe can be fromabout 260 to about 315° C. (500-600° F.) even though the water is muchcolder. This colder water can absorb more heat per kg (pound) withoutlocalized boiling occurring in the film at the tube wall effecting alower water rate for a given quantity of heat transfer. Boiling in thefilm may cause excessive pressure drop in this water coil. Another wayto attain such tube metal temperature is to cool with highpressure/moderate temperature boiler feed water. In one embodiment, 0.2MW (0.7 MBtu/hr) of heat can be removed via a single serpentine coil (asshown in FIG. 2) in a 4 m (13.5 ft) diameter flash/separation drum,where the coils are 10 cm (4 in) Nominal Pipe Size (NPS) with 2 cm (0.75in) height fins at 0.8 fins/cm (2 fins/in). The embodiment uses 4500kg/hr (10000 lbs/hr) of 10500 kPa (1500 psig) boiler feed water heatedfrom about 150 to about 180° C. (300-360° F.) with a maximum filmtemperature, i.e., the maximum temperature of the water in contact withthe pipe walls (with no localized boiling and flow cycling) of about240° C. (460° F.). Maximum tube metal temperature (TMT) is about 255° C.(490° F.) while maximum fin tip temperature is about 350° C. (660° F.).

The vapor phase taken as overhead from the flash/separation apparatus 5via 13 may contain, for example, 55 to 70% hydrocarbons and 30 to 45%steam. The boiling end point of the vapor phase is normally below about760° C. (1400° F.), such as below about 1100° F. (590° C.). The vaporphase is continuously removed from the flash/separator vessel 5 throughan overhead pipe, which optionally conveys the vapor to a centrifugalseparator 38 to remove trace amounts of entrained and/or condensedliquid which can be recycled to boot 35 as quench via line 39.Optionally, steam cracker gas oil (about 205 to about 290° C. (400 to560° F.) boiling range) or other low viscosity hydrocarbon having asimilar boiling range can be added to line 39 as quench or fluxant. Thevapor from line 13 then typically flows into a manifold that distributesthe flow to the convection section of the furnace.

The vapor phase stream 13 continuously removed from the flash/separatorvessel is preferably superheated in the pyrolysis furnace lowerconvection section 23 to a temperature of, for example, about 425 toabout 705° C. (800 to about 1300° F.) by the flue gas from the radiantsection of the furnace. The vapor phase is then introduced to theradiant section of the pyrolysis furnace to be cracked.

The vapor phase stream 13 removed from the flash/separator vessel canoptionally be mixed with a bypass steam stream 21 before beingintroduced into the furnace lower convection section 23.

The bypass steam stream 21 is a split steam stream from the secondarydilution steam 18. Preferably, the secondary dilution steam is firstheated in the convection section of the pyrolysis furnace 3 beforesplitting and mixing with the vapor phase stream removed from the flash5. In some applications, it may be possible to superheat the bypasssteam again after the splitting from the secondary dilution steam butbefore mixing with the vapor phase. The superheating after the mixing ofthe bypass steam 21 with the vapor phase stream 13 ensures that all butthe heaviest components of the mixture in this section of the furnaceare vaporized before entering the radiant section. Raising thetemperature of vapor phase from about 425 to about 705° C. (800 to 1300°F.) in the lower convection section 23 also helps the operation in theradiant section since radiant tube metal temperature can be reduced.This results in less coking potential in the radiant section. Thesuperheated vapor is then cracked in the radiant section of thepyrolysis furnace.

Because the controlled flash of the mixture stream results insignificant removal of the coke- and tar-producing heavier hydrocarbonspecies (in the liquid phase), it is possible to utilize a transfer lineexchanger for quenching the effluent from the radiant section of thepyrolysis furnace. Among other benefits, this will allow morecost-effective retrofitting of cracking facilities initially designedfor lighter feeds, such as naphthas, or other liquid feedstocks with endboiling points generally below about 315° C. (600° F.), which havetransfer line exchanger quench systems already in place.

After being cooled in the transfer line exchanger, the furnace effluentmay optionally be further cooled by injection of a stream of suitablequality quench oil.

The present invention's use of an internal partial condenser within theflash/separation apparatus provides several benefits. The condensercleans up during each steam/air decoke of the drum, eliminating costlymaintenance and shutdowns. The condenser's minimal space requirementspermit retrofitting of current flash/separation apparatus. Where foulingis caused by entrainment of resid rather than strictly vapor/liquidequilibrium, the raining droplets produced by the condenser will alsoremove liquid resid in the vapor. Where a 50% reduction is achieved inthe 760° C. (1400° F.) or above fraction present in the vapor exitingthe flash/separation apparatus, overhead fouling is reduced or a greaterhydrocarbon vapor cut can be taken.

While the present invention has been described and illustrated byreference to particular embodiments, those of ordinary skill in the artwill appreciate that the invention lends itself to variations notnecessarily illustrated herein. For this reason, then, reference shouldbe made solely to the appended claims for purposes of determining thetrue scope of the present invention.

1. A process for cracking a hydrocarbon feedstock containing resid, saidprocess comprising: (a) heating said hydrocarbon feedstock; (b) mixingthe heated hydrocarbon feedstock with steam to form a mixture stream;(c) introducing the mixture stream in a flash/separation vessel throughan inlet to form i) a vapor phase at its dew point which contains alesser portion of coke precursors, and ii) a liquid phase which containsa greater portion of coke precursors; (d) partially condensing saidvapor phase within said flash/separation vessel by contacting said vaporphase with a condenser, which condenses at least some of said lesserportion of coke precursors, which adds to said liquid phase, saidcondensing providing a vapor phase above the condenser of reduced cokeprecursors content; (e) removing the vapor phase of reduced cokeprecursors content as overhead through an overhead outlet, and saidliquid phase as bottoms, from said flash/separation vessel; (f) heatingthe vapor phase; (g) cracking the heated vapor phase in a radiantsection of a pyrolysis furnace to produce an effluent comprisingolefins, said pyrolysis furnace comprising a radiant section and aconvection section; and (h) quenching the effluent and recoveringcracked product therefrom.
 2. The process of claim 1, wherein saidcondenser is located above said inlet of the flash/separation vessel. 3.The process of claim 2 wherein said condenser comprises aheat-conducting tube containing a cooling medium.
 4. The process ofclaim 3 wherein said tube is metal.
 5. The process of claim 4 whereinsaid cooling medium comprises liquid.
 6. The process of claim 5 whereinsaid cooling medium comprises water.
 7. The process of claim 6 whereinsaid condenser is substantially planar and horizontally mounted in saidflash/separation vessel.
 8. The process of claim 7 wherein said tube iscontinuous and comprised of alternating straight sections and 180° bendsections beginning with a straight inlet section and terminating in astraight outlet section.
 9. The process of claim 7 wherein saidcondenser comprises a substantially straight inlet communicating with aninlet manifold substantially perpendicular to said straight inlet, atleast two parallel cooling tubes substantially perpendicular to andcommunicating with said inlet manifold and substantially perpendicularto and communicating with an outlet manifold, and a substantiallystraight outlet perpendicular to and communicating with said outletmanifold.
 10. The process of claim 4 wherein said tube has an outsidetube metal temperature (TMT) ranging from about 200 to about 370° C.(400 to 700° F.).
 11. The process of claim 10 wherein said tube has anoutside tube metal temperature (TMT) ranging from about 260 to about315° C. (500 to 600° F.).
 12. The process of claim 3 wherein said tubehas a diameter of about 2.5 to 10 cm (1 to 4 in).
 13. The process ofclaim 3 wherein said condenser heat duty ranges from at least one of i)about 0.06 to about 0.60 MW (0.2 to 2 MBtu/hr) and ii) about 0.06 toabout 0.6% of furnace firing.
 14. The process of claim 3 wherein saidcondenser heat duty ranges from at least one of i) about 0.1 to about0.3 MW (0.4 to 1 MBtu/hr) and ii) about 0.1 to about 0.3% of furnacefiring.
 15. The process of claim 8 wherein said cooling medium is boilerfeed water.
 16. The process of claim 15 which further comprises passingsaid boiler feed water through said condenser at a rate of about 450 toabout 13000 kg/hr (1 to 30 klb/hr) at a temperature ranging from about100 to about 260° C. (212 to 500° F.), at a pressure ranging from about350 to about 17,000 kPag (50 to 2500 psig).
 17. The process of claim 3wherein fins are attached to said tube along its length.
 18. The processof claim 17 wherein said tube comprises at least about 2 fin/cm of tubelength (5 fins/inch of tube length).
 19. The process of claim 18 whereinsaid fins range from about ⅝ to about 2½ cm (¼ to 1 in) in height, andabout 0.05 to about 0.4 cm (0.02 to 0.15 in) in thickness.
 20. Theprocess of claim 19 wherein said fins are of about 2 cm (¾ in) inheight, and about ⅛ cm (0.05 in) in thickness.
 21. The process of claim3 wherein said tube comprises a substantially concentrically placedinner tube within an outer tube, wherein water is passed through saidinner tube and steam is passed through said outer tube.
 22. The processof claim 21 wherein said inner tube has a diameter ranging from about 2½to about 10 cm (1 to 4 in) and said outer tube has a diameter rangingfrom about 5 to about 15 cm (2 to 6 in).
 23. The process of claim 22wherein said inner tube has a diameter of about 5 cm (2 in) and saidouter tube has a diameter of about 10 cm (4 in).
 24. The process ofclaim 3 wherein a set of passive liquid/vapor contacting surfaces ispositioned beneath said condenser to improve contact between condensateand rising vapor.
 25. The process of claim 24 wherein said set ofliquid/vapor contacting surface(s) is provided by a first row of shedsarranged substantially perpendicularly to said tube.
 26. The process ofclaim 25 wherein said set of liquid/vapor contacting surfaces furthercomprises at least one additional row of sheds substantially parallel toand beneath said first row of sheds.
 27. The process of claim 24 whereina second condenser is located beneath said liquid/vapor contactingsurfaces.
 28. The process of claim 1 wherein said mixture stream isintroduced to the flash/separation vessel through a tangential inletthrough a side of said vessel.
 29. The process of claim 3 wherein saidtube comprises a tube inlet and a tube outlet.
 30. The process of claim29 wherein at least one of said tube inlet and said tube outlet passthrough a wall of said flash/separation vessel.
 31. The process of claim29 wherein at least one of said tube inlet and said tube outlet passthrough said overhead outlet of said flash/separation vessel.
 32. Theprocess of claim 1 wherein said condensing provides a vapor phasereduced in coke precursor content by at least about 50% relative to acomparable vapor phase produced in the absence of said condensing. 33.The process of claim 32 wherein said condensing provides a vapor phasereduced in coke precursor content by at least about 80% relative to acomparable vapor phase produced in the absence of said condensing. 34.The process of claim 1 wherein said mixture stream is heated prior tointroducing to said flash/separation vessel.
 35. A flash/separationvessel for treating hydrocarbon feedstock containing resid to provide aliquid phase and a vapor phase, which comprises: (a) an inlet forintroducing to said vessel under flashing conditions a mixture streamcomprising said hydrocarbon feedstock and steam where the mixture streamundergoes an initial flashing to form i) a vapor phase at its dew pointwhich contains a lesser portion of coke precursors; and ii) a liquidphase which contains a greater portion of coke precursors; (b) a partialcondenser for contacting the vapor phase within said flash/separationvessel and at least partially condensing at least some of said lesserportion of coke precursors, which adds to said liquid phase, saidcondensing providing a vapor phase of reduced coke precursor content;(c) a flash/separation vessel overhead outlet for removing the vaporphase of reduced coke precursors content as overhead; and (d) aflash/separation vessel liquid outlet for removing said liquid phase asbottoms from said flash/separation vessel.
 36. The vessel of claim 35wherein said condenser is located above said inlet of theflash/separation vessel.
 37. The vessel of claim 36 wherein saidcondenser comprises a heat-conducting tube containing a cooling medium.38. The vessel of claim 37 wherein said tube is metal.
 39. The vessel ofclaim 38 wherein said cooling medium comprises liquid.
 40. The vessel ofclaim 39 wherein said cooling medium comprises water.
 41. The vessel ofclaim 39 wherein said condenser is substantially planar and horizontallymounted.
 42. The vessel of claim 41 wherein said tube is continuous andcomprised of alternating straight sections and 180° bend sectionsbeginning with a straight inlet section and terminating in a straightoutlet section.
 43. The vessel of claim 41 wherein said condensercomprises a substantially straight inlet communicating with an inletmanifold substantially perpendicular to said straight inlet, at leasttwo substantially parallel cooling tubes substantially perpendicular toand communicating with said inlet manifold and substantiallyperpendicular to and communicating with an outlet manifold, and asubstantially straight outlet perpendicular to and communicating withsaid outlet manifold.
 44. The vessel of claim 37 wherein said tube has adiameter of about 2.5 to about 10 cm (1 to 4 in).
 45. The vessel ofclaim 37 wherein said condenser heat duty ranges from at least one of i)about 0.06 to about 0.60 MW (0.2 to 2 MBtu/hr) and ii) about 0.06 toabout 0.6% of furnace firing.
 46. The vessel of claim 45 wherein saidcondenser heat duty ranges from at least one of i) about 0.1 to about0.3 MW (0.4 to 1 MBtu/hr) and ii) about 0.1 to about 0.3% of furnacefiring.
 47. The vessel of claim 40 wherein said cooling medium is boilerfeed water.
 48. The vessel of claim 37 wherein fins are attached to saidtube along its length.
 49. The vessel of claim 48 wherein said tubecomprises at least about 2 fin/cm of tube length (5 fins/inch of tubelength).
 50. The vessel of claim 49 wherein said fins range from about ⅝to about 2½ cm (¼ to 1 in) in height, and about 0.05 to about 0.4 cm(0.02 to 0.15 in) in thickness.
 51. The vessel of claim 50 wherein saidfins are of about 2 cm (¾ in) in height, and about ⅛ cm (0.05 in) inthickness.
 52. The vessel of claim 37 wherein said tube comprises asubstantially concentrically placed inner tube within an outer tubewherein water is passed through said inner tube and steam is passedthrough said outer tube.
 53. The vessel of claim 52 wherein said innertube has a diameter ranging from about 2½ to about 10 cm (1 to 4 in) andsaid outer tube has a diameter ranging from about 5 to about 15 cm (2 to6 in).
 54. The vessel of claim 53 wherein said inner tube has a diameterof about 5 cm (2 in) and said outer tube has a diameter of about 10 cm(4 in).
 55. The vessel of claim 37 wherein a set of passive liquid/vaporcontacting surfaces is positioned beneath said condenser to improvecontact between condensate and rising vapor.
 56. The vessel of claim 55wherein said set of liquid/vapor contacting surface(s) is provided by afirst row of sheds arranged substantially perpendicular to said tube.57. The vessel of claim 56 wherein said set of liquid/vapor contactingsurfaces further comprises at least one additional row of shedssubstantially parallel to and beneath said first row of sheds.
 58. Thevessel of claim 55 wherein a second condenser is located beneath saidliquid/vapor contacting surfaces.
 59. The vessel of claim 35 whereinsaid inlet is a substantially tangential inlet through a side of saidvessel.
 60. The vessel of claim 37 wherein said tube comprises a tubeinlet and a tube outlet.
 61. The vessel of claim 60 wherein at least oneof said inlet and said outlet pass through a wall of saidflash/separation vessel.
 62. The vessel of claim 60 wherein at least oneof said tube inlet and said tube outlet pass through said overheadoutlet of said flash/separation vessel.
 63. An apparatus for cracking ahydrocarbon feedstock containing resid, said apparatus comprising: (a) aheating zone for heating said hydrocarbon feedstock to provide heatedhydrocarbon feedstock; (b) a mixing zone for mixing a primary dilutionsteam stream with said heated hydrocarbon feedstock to provide a heatedtwo-phase stratified open channel flow mixture stream; (c) avapor/liquid separation zone for treating vapor/liquid mixture streamsof hydrocarbons and steam, said zone comprising: (i) a substantiallycylindrical vertical drum having an upper cap section, a middle sectioncomprising a circular wall, and a lower cap section; (ii) an overheadvapor outlet attached to said upper cap section; (iii) at least onesubstantially tangentially positioned inlet in the wall of said middlesection for introducing said flow mixture stream along said wall underflashing conditions where the flow mixture stream undergoes an initialflashing to form A) a vapor phase at its dew point which contains alesser portion of coke precursors, and B) a liquid phase which containsa greater portion of coke precursors; (iv) a partial condenser forcontacting the vapor phase within said drum for at least partiallycondensing at least some of said lesser portion of coke precursors,which adds to said liquid phase, said condensing providing a vapor phaseof reduced coke precursors content; (v) a drum overhead outlet forremoving the vapor phase of reduced precursors content as overhead; (vi)a drum liquid outlet for removing said liquid phase as bottoms from saiddrum; and (vii) a substantially concentrically positioned, substantiallycylindrical boot of less diameter than said middle section, said bootcommunicating with said lower cap section, and further comprising aninlet for quench oil and a liquid outlet at its lower end; (d) apyrolysis furnace comprising a convection section, and a radiant sectionfor cracking the vapor phase from the overhead vapor outlet to producean effluent comprising olefins; (e) a means for quenching the effluent;and (f) a recovery train for recovering cracked product from thequenched effluent.
 64. The apparatus of claim 63 wherein said condenseris located above said inlet of the drum.
 65. The apparatus of claim 64wherein said condenser comprises a heat-conducting tube containing acooling medium.
 66. The apparatus of claim 65 wherein said tube ismetal.
 67. The apparatus of claim 66 wherein said cooling mediumcomprises liquid.
 68. The vessel of claim 67 wherein said cooling mediumcomprises water.
 69. The apparatus of claim 63 wherein said condenser issubstantially planar and horizontally mounted.
 70. The apparatus ofclaim 65 wherein said tube is continuous and comprised of alternatingstraight sections and 180° bend sections beginning with a straight inletsection and terminating in a straight outlet section.
 71. The apparatusof claim 65 wherein said condenser comprises a substantially straightinlet communicating with an inlet manifold substantially perpendicularto said straight inlet, at least two substantially parallel coolingtubes substantially perpendicular to and communicating with said inletmanifold and substantially perpendicular to and communicating with anoutlet manifold, and a substantially straight outlet perpendicular toand communicating with said outlet manifold.
 72. The apparatus of claim63 wherein said quench oil is recycle quench oil.
 73. The apparatus ofclaim 63 wherein a set of passive liquid/vapor contacting surfaces ispositioned beneath said condenser to improve contact between condensateand rising vapor.
 74. The apparatus of claim 63 which further comprisesa heating zone for heating the mixture stream upstream of theflash/separation zone.